For the same reason we aren't burning anything else in a fusion reaction. We don't know how. At least not in a way that produces more power than it takes to cause the reaction. Yet.

That's not really the hitch: you don't care about power consumption, since you can always haul your own fission plant along with you. The reason we don't have a drive is that we're never able to keep the reaction going for more than a few seconds -- I think the record *might* be a minute, but I'm not sure.

The reason we don't have a drive is that we're never able to keep the reaction going for more than a few seconds -- I think the record *might* be a minute, but I'm not sure.

As far as I know, all past reactions have been done with deuterium and tritium, because they react at a lower temperature. This is also the reason helium3 is desirable. In the few minutes of googling I have just done, the reason usually given for preferring decaborane as a fuel is that most of the energy comes out in charged particles and not neutrons. However, it ignites at a higher temperature, so I don't believe any sustained reaction using that fuel has been achieved yet.

when used in Boron-11/Hydrogen fusion has an exhaust velocity of 11800000 m/s (specific impulse of over 1.2 million seconds).

With a 1g burn, it would take 4 days 13 hours to get to Mars on furthest approach; a mere 2 days 1.5 hours on closest approach. For the furthest approach, ~28% of the spacecraft would be fuel; closest, ~14%.

These are the fastest times possible with any propulsion technology (not counting warp drives), as they entail accelerating at 1g until half way then turning around and decelerating at 1g for the other half.

This implies that your ship will have a thrust to weight of 1 (or thrust to mass of 9.8N/kg if you prefer.) If your whole ship masses a ton you will need to produce 9800N thrust at a Ve of 11,800,000 which implies 0.8g/s of propellant. Your exhaust will be helium at a temperature of about 100,000,000K. Now, I don't know if the ideal gas law extends to those temperatures, but if it does 0.8g of helium at that temperature would have a volume of ~1600m^3 at one atmosphere pressure. Your engine will have to be huge or high pressure, or both. It will not be light. I guess I'm comfortable with my prediction of 19 days, although the method might be fusion drives at less than 1g acceleration.